The USB-C monitor only turns on if the power supply is not connected.

Question

Level 1

4 points

The USB-C monitor only turns on if the power supply is not connected.

Good morning, I have a MacBook Pro M1, a MacBook Pro M2, and a Mac Mini M1, and all of them have issues when I try to use the USB-C output on monitors.

With both MacBooks, if I connect them using only the USB-C cable, the monitor turns on, and everything works normally. But if I plug in the power supply, the external monitor turns off and only turns back on if I unplug and replug the USB-C cable. Sometimes, I have to plug it in very slowly; otherwise, it won't work.

As for the Mac Mini, it starts up with the monitors working, but after a few minutes, the monitor connected via USB-C turns off. Sometimes, turning the monitor off and on again solves the issue, but in many cases, I need to disconnect and reconnect the USB-C cable. This happens frequently.

It’s worth noting that this doesn’t happen when I use the HDMI output.

It seems to be something related to the monitor’s power consumption.

I’ve already tried lowering the monitor's refresh rate, but that didn’t help.

Does anyone know why this happens?

MacBook Pro 16″, macOS 15.0

Posted on Oct 17, 2024 6:15 AM

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Answer 1

Oct 17, 2024 2:52 PM in response to Grant Bennet-Alder

I think the problem is your interpretation of USB-C power delivery. if you want the display to provide power in the usual way at up to 60 Watts, !00 Watts (or even up to 240 watts with a super premium cable) you need to be using a genuine USB-C cable with that power delivery rating of 60, 100, or 240 Watts.

ThunderBolt cables are somewhat different, but when it comes to supplying power, ThunderBolt cables are COMPLETLY different, and only allow at most 18 watts to be passed over the cable, and completely flummoxed when the peripheral is the device trying to supply the power.

To obtain predictable results, use a genuine USB-C cable, because Thunderbolt cables are not the same.

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Answer 2

Oct 17, 2024 12:52 PM in response to sidaoBR

LG appears to be a 2K display with

INPUT/OUTPUT

HDMI

Yes (ver1.4)

DisplayPort

Yes (ver1.2)

USB-C

Yes (DP Alt. Mode)

Samsung appear to be a full HD display with:

Inputs
VGA
1
HDMI
1
D-Sub
1
HDMI Version
1.4

I see no evidence that this display has USB input or USB power delivery.

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Answer 3

sidaoBR Author

Level 1

4 points

Oct 17, 2024 1:34 PM in response to Grant Bennet-Alder

I use the usb-c to connect to a display, not to charge my laptop, so I want just use the usb-c to use second monitor.

In my macmini the HDMI port connect to a monitor and I'm trying using the usb-c to connect another display.

Both display are self charge using the own power supply, not necessary the power from macmini or MacBook.

But I don't know why this issue happen.

For example, how I explained before, MacBooks when turn on without power charge (mega safe) just plugged to the second monitor it's work perfectly, but, laptop discharge of corse.

When I plug the power on MacBook display turn off, I disconnect and reconnect the usb-c cable and all works very well.

If I turn on de MacBook with both, plug charge and usb-c the second monitor not turn on, just if I disconnect and reconnect the usb-c cable.

So, in macmini the second display using usb-c turn off frequently, what is very strange, because display is power by own charge.

How can I use two display in macmini? I think the usb-c needs do it.

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Answer 4

Oct 17, 2024 7:21 AM in response to sidaoBR

what make&model display, or a link to its specs?

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Answer 5

sidaoBR Author

Level 1

4 points

Oct 17, 2024 12:06 PM in response to Grant Bennet-Alder

I'm using a LG ultra wide 29UM69G-B or a Samsung LF24T with a UGOURD thunderbolt 4 cable.

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The USB-C monitor only turns on if the power supply is not connected.

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